A bridge has been provided as an apparatus for measuring reflection characteristics of a device under test (DUT).
FIG. 6 is a circuit diagram of a conventional bridge for measuring of reflection-characteristics. In FIG. 6, the bridge comprises a bridge circuit 4 and a balanced signal source 5, and is designed as follows.
In the bridge circuit 4, a series circuit of resistors Rb and Rc (the connection point between Rb and Rc is represented by p) and a series circuit of resistor Ra and DUT (the connection point between Ra and DUT is represented by q) are connected in parallel, and a measurement instrument 6 (internal resistance R.sub.o) is connected between p and q of the parallel-connection circuit. The connection point q is connected to the ground (GND). The connection point n between DUT and Rc and the connection point m between Rb and Ra are supplied with a balanced signal from a balanced signal source 5.
The balanced signal source 5 comprises a signal source V.sub.1 having an internal resistance R.sub.1 and a transformer, T.sub.1. An unbalanced signal from the signal source V.sub.1 is applied to a primary winding of the balance/unbalance converting transformer T.sub.1 whose one terminal is connected to the ground, and a balanced signal is taken out of a secondary winding.
The balanced signal source 5 has typically used a circuit using the transformer T.sub.1 as shown in FIG. 6 in which primary and secondary windings are separated and one terminal of the primary winding is grounded, a circuit using a transmission-line type of transformer T.sub.2 as shown in FIG. 7, or a circuit using the combination of the above circuits. Particularly in a reflection-characteristics measuring bridge which is used in combination with a network analyzer, such as, 87511A Network Analyzer manufactured and sold by Hewlett-Packard Company, the circuit using the transformer T.sub.2 as shown in FIG. 7 has been frequently used in consideration of its high-frequency characteristic.
When the balanced signal source uses the transformer T.sub.1 as described above, an frequency band has a lower limit at a frequency band and cannot be used with a DC current. If its characteristic is lower than several tens Hz, the transformer T.sub.1 must be disavantageously large in size and heavy in weight.
Further, it is natural that a low-frequency transformer has a deteriorated frequency characteristic, and its frequency bandwidth is usually about four digits. Therefore, conventionally, a special consideration must be paid in order to broadening an bandwidth over the limited bandwidth.
For example, when a reflection wave is measured over a frequency band of about 100 Hz to 10 MHz, the frequency band is divided into plural narrow bands, and a reflection-characteristic measuring bridge with a transformer having the optimum frequency characteristic for each band is individually provided. For measurement of a reflection characteristic of a DUT, a reflection-characteristic measuring bridge which is most suitable for the frequency band is selectively used in accordance with the frequency band.
Therefore, for measurement over a broad frequency band, plural reflection-characteristic measuring bridges are required, and thus not only the number of parts is disadvantageously increased, but also a device is required to be designed in large size.
In a broadcasting station and a recording studio, a balanced output type amplifying circuit 21 as shown in FIG. 8 is used to depress an induced noise to a cable (see Japanese Laid-open Utility Model Application No. 57-166412).
The amplifying circuit 21 as described above converts a signal from a signal source in an unbalanced system (not shown) to a balanced signal.
That is, the unbalanced signal is input from an input terminal Pi to a buffer amplifying circuit 22. The buffer amplifying circuit 22 supplies its output signal to an amplifying circuit 24 and at the same time supplies its output signal through an inverting amplifying circuit 23 to an amplifying circuit 25.
The amplifying circuit 22 comprises an operational amplifying circuit OPa, and the amplifying circuit 23 comprises an operational amplifier OPb and resistors r.sub.31 and r.sub.32. The amplifying circuit 24, 25 comprises an operational amplifying circuit OPc and resistors r.sub.41, r.sub.42, r.sub.43, r.sub.44 and r.sub.45, or an operational amplifying circuit OPd and resistors r.sub.51, r.sub.52, r.sub.53, r.sub.54 and r.sub.55.
The amplifying circuit 25 is subjected to a feedback range from the output of the amplifying circuit 24 through the resistor r.sub.55, and the amplifying circuit 24 is subjected to the feedback from the output of the amplifying circuit 25 through the resistor r.sub.45. A balanced output signal is output from the output terminal Pa of the amplifying circuit 24 and the output terminal Pb of the amplifying circuit 25.
The balanced output type amplifying circuit 21 is usable in place of the balanced signal source 5 as described with reference to FIG. 6, and the measurement for a frequency range includes a direct-current area.
However, such an amplifier aims to cover a voice band, and thus the upper limit frequency thereof ranges from several tens kHz to several hundreds kHz. Therefore, this amplifier has a disadvantage that the frequency band can not be broadened.